When I was a lot younger I had a car that I loved and had rebuilt myslef. I always kept track on the mileage at every fill up. If it suddenly went down I knew that I had a problem somewhere and needed to check it out. Of course this was back in the days of carburetors. These days, I'm old and I just read whatever the dash display tells me and figure it's close enough for me.
I understand why people driving RV's across the country might want keep a close eye on it. If I was living that lifestyle I think I would probably write it down by hand too.